1. Field of the Invention
The present invention relates to a method of press-molding a ceramic green sheet laminate, for pressing a ceramic green sheet laminate which is formed by stacking a plurality of ceramic green sheets with each other along its thickness thereby molding the same, and more particularly, it relates to a method of press-molding a ceramic green sheet laminate with an improved pressure profile for pressing the ceramic green sheet laminate along its thickness.
2. Description of the Background Art
In order to manufacture ceramic multilayer electronic parts such as multilayer capacitors, for example, a plurality of mother ceramic green sheets printed with internal electrode materials are stacked with each other and a proper number of unprinted mother ceramic green sheets are stacked on the uppermost and lowermost ones of the printed ceramic green sheets, thereby obtaining a ceramic green sheet laminate. Thereafter the ceramic green sheet laminate is pressed along its thickness so that the ceramic green sheets are compression-bonded to each other and the laminate is molded. Then, the mother laminate thus obtained is cut along its thickness, to obtain laminate raw chips in units of individual ceramic multilayer electronic parts. The laminate raw chips thus obtained are fired to obtain ceramic sintered bodies, which in turn are provided with external electrodes on outer surfaces thereof.
In the aforementioned manufacturing method, the ceramic green sheet laminate is pressed along its thickness to be press-molded since it is impossible to obtain ceramic sintered bodies consisting of densely sintered ceramics, and layer breakage such as delamination is caused unless the ceramic green sheets are sufficiently brought into close contact with each other. Thus, the mother ceramic green sheet laminate is subjected to the aforementioned pressing step so that the ceramic green sheets strongly adhere to each other.
An example of the aforementioned method of press-molding a ceramic green sheet laminate is described with reference to FIGS. 1 and 2.
A lower mold section 1 having a cavity 1a and an upper mold section 2 are employed for press-molding the ceramic green sheets. A plurality of green sheets 3 are introduced into the cavity 1a of the lower mold section 1, and stacked with each other. Thereafter the upper mold section 2 which is sized to be receivable in the cavity 1a is placed on the ceramic green sheet laminate as formed, and a pressure P is applied from above for press-molding the ceramic green sheet laminate. The lower mold section 1 is mounted on a lower ram 4 of a press, while the upper mold section 2 is mounted on an upper ram 5 thereof. FIG. 2 shows a pressure profile obtained in practice. A pressure was gradually applied from the upper mold section 2 so that the ceramic green sheet laminate was pressed at the pressure level P for a prescribed time T after a lapse of a time t.sub.1, and thereafter the pressure was gradually reduced over a time t.sub.2, thereby pressing the ceramic green sheet laminate. The pressure value P was about 350 tons in general, while the times t.sub.1, T and t.sub.2 were about 120 to 180 seconds, about 60 seconds and about 120 to 180 seconds respectively.
In the aforementioned method of press-molding a ceramic green sheet laminate, it is necessary to apply constant force per unit area, in order to reliably compression-bond the ceramic green sheets 3 with each other. Therefore, the pressure P as required must be increased as the area of each ceramic green sheet 3 is increased.
When the pressure P is increased, however, the overall dimensions of the press provided with the lower mold section 1 and the upper mold section 2 must inevitably be increased. Thus, it is necessary to employ a high-priced press which requires a large space.
When the pressure P is increased, further, distortion which is applied to the ceramic green sheets 3 in response thereto is inevitably increased. In manufacturing ceramic multilayer electronic parts such as multilayer capacitors, therefore, positions of internal electrodes which are printed on the ceramic green sheets are displaced due to such distortion, and hence it is difficult to obtain ceramic electronic parts having desired characteristics.